1. Field of the Invention
The present invention relates to a semiconductor device having a circuit structured by a thin film transistor (hereinafter referred to as a TFT) on a substrate having an insulating surface, and a method of manufacturing the semiconductor device. In particular, the present invention relates a formation method of a crystalline semiconductor film used as an active layer of a TFT. Note that in this specification, a semiconductor device indicates a general device functioning by utilizing semiconductor characteristics, and semiconductor devices include electro-optical devices and electronic equipment loaded with an electro-optical device.
2. Description of the Related Art
The development of semiconductor devices having a large surface area integrated circuit formed by TFTs on a substrate having an insulating surface has been advancing. Active matrix type liquid crystal display devices, EL display devices, and contact type image sensors are known as typical examples of such. In particular, a TFT (hereinafter, a polysilicon TFT is used as a typical example) in which a crystalline semiconductor film (typically a polysilicon film) is made into an active layer has a high electric field effect mobility, and therefore it is possible to use it to form many kinds of function circuits.
For example, in an active matrix type liquid crystal display device, integrated circuits such as the following are formed on one substrate for each functional block: a pixel circuit for performing image display; and a shift register circuit based on CMOS circuits; a level shifter circuit; a buffer circuit; and a sampling circuit. Further, driver circuits for controlling a pixel circuit, such as a sample hold circuit, a shift register circuit, and a multiplexer circuit, are formed in a contact type image sensor using a TFT. Therefore, depending upon their usage objective, a high degree of element performance is required for TFTs.
The performance of the polysilicon TFT largely follows the characteristics of the polysilicon film used as an active layer. Therefore, the manufacture of a good quality polysilicon film by a simple process is of great importance in the manufacturing process of the polysilicon TFT.
The polysilicon film used in the active layer is normally formed by the crystallization, through some method, of an amorphous silicon film. Specifically, this is performed by a process such as laser irradiation, lamp annealing using an infrared light, or furnace annealing with an electric furnace. Of these, when crystallization is performed by electric furnace heat treatment, normally a high temperature of 600xc2x0 C. or greater is necessary. Note that below 600xc2x0 C., crystallization is extremely slow, and is impractical.
However, if the process temperature is high, then it is necessary to use a high cost material such as quartz for the substrate, and there is a disadvantage in which the costs become high. Therefore, a technique of crystallization at a rather low temperature is required. For this end, it is effective to use catalytic elements which promote crystallization. For example, it is disclosed in Japanese Patent Application Laid-Open No. Hei 7-130652 (corresponding to U.S. Pat. No. 5,643,826) that low temperature crystallization is possible by doping a catalyst, typically Ni, into the amorphous silicon film.
However, a harmful affect may be imparted to the TFT performance when Ni remains within the active layer. Gettering is then necessary by some type of method after crystallization is completed. Methods such as a method of doping phosphorous at a high concentration outside the region which becomes a channel and then moving Ni in a horizontal direction by heat treatment, and a method of drawing out the catalyst in the liquid phase by immersion in hot sulfuric acid can be considered as gettering methods.
However, the above method have the following problems: a relatively long heat treatment time is necessary for gettering the Ni by using phosphorous, and the gettering efficiency by the liquid phase method with sulfuric acid is insufficient, resulted in a problem for the process. Thus an object of the present invention is the development of a process in which a polysilicon film is formed using a catalyst that is easily gettered, and with which crystallization can be made at a low temperature to manufacture a desired polysilicon TFT using the polysilicon film.
In order to solve the above problems, according to an aspect of the present invention, the gettering of the Cu within the semiconductor film is performed by the steps of:
introducing a Cu catalyst into an amorphous semiconductor film by performing spin coating of a solution containing Cu on the amorphous semiconductor film;
polycrystallizing the amorphous semiconductor film by heat treatment in an electric furnace; and
immersing the completed polycrystalline semiconductor film, containing a small amount of Cu, into at least one chemical solution selected from the following chemical solutions: a chemical solution containing oxygen (such as sulfuric acid, nitric acid, oxalic acid, or aqua regia), and a chemical solution which does not contain oxygen (such as hydrochloric acid or hydrofluoric acid).
Further, an aqueous solution of copper chloride (CuCl2) dissolved in dilute hydrochloric acid is used as the solution containing Cu in the above process. The Cu concentration when used is from 1 to 1000 ppm by weight, and hydrochloric acid is between 0.01 and 0.1%.
Furthermore, a metallic Cu thin film may be formed by a method such as sputtering in the above process as a substitute for spin coating the solution on the amorphous semiconductor film.